The invention relates to a modified reaction resin, a process for its production and also its use for the production of shaped thermosetting plastics with improved fracture resistance, especially impact resistance.
Starting products or prepolymers which are to be understood as addressed in the scope of the present invention are from among the reaction resins which are liquid or plastic before and during the processing and shaping process and following the traditional shaping processing, as a result of polyreaction (polymerization, polycondensation, polyaddition), yield thermosetting plastics. A three-dimensional, crosslinked, hard, non-melting resin, the thermosetting plastic, is obtained by the polyreaction, and the thermosetting plastic thus differs basically from traditional thermoplastic plastics, which as is known can be liquified and/or plasticized repeatedly by reheating.
As a result of the generally very high density of crosslinking, the crosslinked reaction resins have a number of valuable properties, which provide the reason that they, along with the thermoplastics, are the most used polymers. These valuable properties especially include hardness, strength, chemical resistance and temperature durability. Because of these properties, these reaction resins are used for various purposes, for instance for the production of fiber-reinforced plastics, for insulation materials used in electrotechnology, for the production of structural adhesives, laminated plastics, annealing lacquers and the like.
In addition to these advantageous properties, the thermosetting plastics have one serious drawback, which in many cases quite prevents their use. As a result of the highly crosslinked state, they are very brittle and have a low impact resistance. This appears especially in the range of low temperatures, in other words at temperatures below 0.degree. C., so that, for uses wherein the polymer is to be subjected to high mechanical stresses at low temperatures, especially impact stresses, the thermoplastic polymers generally have the advantage, whereby the drawbacks connected therewith, such as lower heat deformation resistance and chemical resistance, must be taken into consideration.
Since this condition is not particularly favorable, there have been many attempts in the past to improve the impact resistance or flexibility of thermosetting plastics.
Thus, it is already known, for instance, to mix reaction resins with fiber fillers, in order to increase the impact resistance. The improvements which are thus obtained are nonetheless quite limited. The addition to resins of powdered, soft filler material, such as powdered rubber or soft elastic plastic powder, is also known. The particle dimensions of such powdered additives is in the range of approximately 0.04 to 1 mm, which obviously does not suffice to improve such reaction resins to the desired degree, and which therefore enhance the drawbacks relative to other important properties required for technical use of this sort of modified thermosetting plastic.
Attempts have been made to improve the impact resistance of crosslinked reaction resins by addition of softeners. The added softeners do not react with the reaction resin, but rather as a result of layering cause a widening of the network of thermosetting plastics and with that a certain softening of the material. A remarkable improvement of the impact resistance can actually be attained in this manner, which however unfortunately results in a limitation of the outlay which is required for the quality of other essential features of the thermosetting plastics. Therefore, with the use of softeners, a latent danger exists of migration occurring following the crosslinking of the reaction resin or with further aging, with the negative results inherent therein for the surface properties of the material, such as the adherence, spreadability, polish and the like.
Furthermore, attempts have also been made to increase the elasticity of thermosetting plastics, in that chain lengtheners are added, which are incorporated into the network with the hardening process and lower the density of crosslinking. Epoxy resins for instance could be elasticized according to this principle by addition of epoxidized soybean oil, dimeric fatty acids or epoxy-functional polyglycol ethers. Since the improvement in elasticity, however, is attained by a decrease, of the crosslinking density, it is connected with the decreasing crosslinking density a deterioration of desirable properties, such as hardness, chemical resistance or temperature durability. This solution therefore also led to results which were not totally satisfactory.
It is also known to use liquid or solid, but uncrosslinked butadiene-acrylonitrile rubbers (nitrile rubber, NBR) as additives to improve the viscosity of the reaction resins. These nitrile rubbers contain functional groups which can be reacted with the reaction resin with the crosslinking process or even in a previous reaction. The remarkable feature of these modifiers as compared with those cited as being used until now resides in that they are actually miscible with the uncrosslinked reaction resin, and a phase separation nonetheless takes place during the crosslinking of the reaction resin, in which the rubber phase is deposited in the form of fine droplets. As a result of the reaction of the functional groups located on the surface of the nitrile rubber particles with the reaction resin, a solid connection of the rubber phase with the thermosetting plastic matrix is formed.
This type of modification of reaction resins is actually more advantageous because the effect is attained not by simply lowering the network density, but rather by formation of a separate soft phase with the result that the other advantageous properties of the thermosetting plastics are not influenced quantitatively by the modifier, as is the case with the measures which were formerly used. Unfortunately, however, such thermosetting plastics modified with nitrile rubber have notable problems. For instance, the heat resistance of thermosetting plastics modified with nitrile rubber is notably decreased and because of that their capacity for use at high temperatures is questionable. This is also true of many electric properties, such as for example the dielectric strength or breakdown resistance. Because of the relatively good compatibility of the nitrile rubber with most reaction resins, especially with epoxy resins, a certain portion of the rubber does not participate in the phase separation during the crosslinking and is incorporated into the resin matrix. The density of crosslinking of the crosslinked reaction resin is thereby lowered with the already noted negative results for the configuration of the properties of the completed thermosetting plastics. Another drawback is the very high viscosity of the nitrile-rubber modifiers, which leads to processing problems and which negatively influences the flow properties of the modified reaction resin.